A .beta.-lactone monomer is a four member cyclic ester having the general formula: ##STR1## They can be unsubstituted, in which case R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are hydrogen, or they can be substituted at any or all of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 with hydrocarbyl, e.g., aliphatic, alicyclic, aromatic groups, or selected reactive functional moieties.
The .beta.-lactone ring displays considerable bond-angle strain. Substituted species further strain the ring by creating steric repulsions. These strains make .beta.-lactones suitable for ring opening polymerization.
Contacting the .beta.-lactone monomer with an initiator will open the ring and cause polymerization to occur. The ring opening is dependent upon the type of initiator used. In the presence of an electrophilic initiator, the acyl-oxygen bond is cleaved. In the presence of a nucleophilic initiator, the alkyl-oxygen bond is cleaved. When a nucleophilic initiator is used, a polyester will form as follows: ##STR2##
U.S. Pat. No. 4,029,718 discloses that certain randomly grafted copolymers of pivalolactone exhibit good resistance to creep and compression set, high tensile strength, high elastic recovery, and good resistance to high temperature deformation. However, ring opening polymerizations of lactones are exothermic and can develop enough heat to decompose unreacted monomer or produce high-viscosity liquid polymers. Reaction temperatures are best kept below 300.degree. C. to avoid these problems.
A number of methods have been used to polymerize .beta.-lactones. For example, see U.S. Pat. Nos. 3,471,456, 3,558,572, 3,578,700, 3,579,489, U.K. Patent No. 1,180,044, U.K. Patent No. 1,121,153 and U.S. Pat. No. 4,988,763. In each of these methods, .beta.-lactones are polymerized in one or more reactors using bulk or slurry processes prior to the fabrication of articles of manufacture. Care must be taken during polymerization to avoid the monomer decomposition and improper polymer formation that can result from the generation of excessive heat. This is difficult in bulk commercial operations because large reaction vessels are difficult to maintain at uniform temperatures. Cooling such reaction vessels containing large quantities of forming polymer can result in localized cooling and inconsistent polymerization. Further, high temperatures such as those found in these processes can terminate living polymerization systems thereby making polymerization incomplete.
Additionally, bulk and slurry polymer production methods require a manufacturer of polymer articles to either produce feed stock in separate polymer plants or to maintain special reactors for producing it. As used herein, the term "feed stock" refers to polymerized .beta.-lactones that are to be used in a further manufacturing step such as thermoplastic molding. In both bulk and slurry methods, feed stock must be transported to a molding apparatus such as a thermoplastic injection mold before fabrication of an item of manufacture can begin. This may require additional pipes and delivery means from the reactor to mold or actual physical delivery of feed stock from one part of a fabrication facility to another.
Thermoplastic molding techniques often employ molds that are heated to high temperatures to ensure that injected polymers remain liquified and flow throughout the mold. Excessive mold heat should be avoided with .beta.-lactone polymers. Generally, this refers to heat in excess of 300.degree. C. Such extreme mold heats will degrade the polymer feed stock and cause the resulting molded product to display poor mechanical properties.
Reaction injection molding (RIM) is a process used for some polymerization reactions in which polymer is formed directly in a mold. Throughout this specification, RIM will be used to refer to any polymerization process in which the polymerization reaction occurs substantially within a mold. Because polymerization occurs directly in the mold, low filling pressures and mold temperatures are involved which allows for molds to be made of less durable, less expensive materials such as aluminum and castable metals. Separate polymer plants are also not needed.
Suitable reactants which are amenable to mold polymerization processes such as RIM must be low viscosity monomers so that they may be easily injected and flow into a mixhead and mold. Viscosity should be from about 50 to about 1000 centipoise. Other polymer constituents such as catalysts, fillers, and pigments must also have low viscosities or must otherwise be capable of transport into the mold without interfering with the flow of monomer. Ideally, such constituents are soluble in the liquid monomer. Liquid flow must also be uniform so that large cavities in molds can be filled without interruption caused by advance polymerization or excessive polymerization. Further, the monomers and other constituents must not volatilize easily because RIM systems are generally closed and have no capacity to release gaseous products. Volatilization also changes the stoichiometry of the reaction making polymer product nonuniform and giving it unpredictable mechanical properties.
The polymer to be produced in mold polymerization processes must also display some particular mechanical properties. Polymer shrinkage cannot be significant or products will lack detail, be deformed or be otherwise incomplete. Polymerization should be quick. Mold times of less than five minutes are most desirable since each product requires a separate polymerization reaction. Lengthy polymerization times increase the number of molds and apparatuses needed to produce significant quantities of a given polymer product. It is a further requirement that polymers used in this process cannot be negatively impacted by the speed of polymerization.
In RIM and related techniques, polymerization acceleration is often aided by exothermic reaction thermodynamics. However, not all exothermic polymerizations will work in RIM conditions. Some reactions are so exothermic that the heat of reaction will degrade the polymer product or the yet unreacted monomer. This is thermal runaway whereby the rate of heat generation overwhelms the rate of heat removal. Even when thermal runaway does not occur, excessive heat may prolong curing time such that the process becomes uneconomical. Thus, a narrow band of exothermic behavior must exist to obtain desirable polymers with these techniques. The reaction must be exothermic enough to enhance acceleration but not so exothermic as to degrade the monomer or forming polymer. The reaction must also not generate so much heat that molding time is unduly lengthened.
Predicting which polymerization reactions will function well under RIM conditions is not readily reducible to theoretical treatment. It is difficult to find monomers and initiators or catalysts which display all of the requisite qualities such as low viscosity, good flow characteristics, low volatility, an absence of shrinkage on polymerization, quick polymerization, and an absence of morphological deformities upon polymerization. Reactions that appear to be attractive candidates for RIM processes are often found to be unsuitable because they possess additional properties that are undesirable. Often, reactions that are found suitable for RIM polymerization are also found to display other unwanted characteristics. For example, dicylopentadiene, a monomer well known in the art to be capable of RIM polymerization, does not display the solvent resistance required for many polymer applications. Moreover, the monomer exudes a strong unpleasant odor. See Encyclopedia of Polymer Science and Engineering, Vol. 14, pg. 89, (Wiley-Interscience, 2 ed. 1987).
Surprisingly, few reactive monomer systems have been found useful in RIM and related techniques because of the lack of predictable behavior and mechanical properties that result. See U.S. Pat. Nos. 4,299,924, 4,426,502 and 5,100,926. See also, Encyclopedia of Polymer Science and Engineering, Vol. 14, pg. 88, for a discussion of RIM polymerization of Nylon.
In the present invention, .beta.-lactones, such as pivalolactone, will open in the presence of an initiator and polymerize under RIM conditions. The polymer formed is a polyester. It can possess a high molecular weight, up to 60,000 [as determined by low angle laser light scattering (LALLS)]. Some polymers thus formed are tough solids and exhibit crystallinity. ##STR3## is a .beta.-lactone monomer that can polymerize to form a particularly tough, crystalline polyester with properties similar to Nylon 6 but with a higher heat distortion temperature ("HDT"). It is also superior to Nylon 6 in its resistance to ultraviolet radiation, ozone, most chemicals and water. Polymers which display these improved properties are useful in a large number of applications including automotive parts, housewares, appliances, electrical components, sporting goods, and numerous other products. Most importantly, they are useful in the manufacture of electronics components such as circuit boards.